In many data communication applications, serializer and de-serializer (SERDES) devices facilitate the transmission of parallel data between two points across a serial link. Data at one point is converted from parallel data to serial data and transmitted through a communications channel to the second point where it received and converted from serial data to parallel data.
At high data rates frequency-dependent signal loss from the communications channel (the signal path between the two end points of a serial link), as well as signal dispersion and distortion, can occur. Ideally, without noise, jitter, and other loss and dispersion effects, a data eye at the receiver will exhibit a relatively ideal shape. In practice, the shape of the data eye changes with noise, jitter, other loss and dispersion effects, and with temperature and voltage variations. As such, the communications channel, whether wired, optical, or wireless, acts as a filter and might be modeled in the frequency domain with a transfer function. Correction for frequency dependent losses of the communications channel, and other forms of signal degradation, often requires signal equalization of the signal at a receiver.
Equalization through use of one or more equalizers compensates for the signal degradation to improve communication quality. Equalization may also be employed at the transmit side to pre-condition the signal. Equalization, a form of filtering, generally requires some estimate of the transfer function of the channel to set its filter parameters. However, in many cases, the specific frequency-dependent signal degradation characteristics of a communications channel are unknown, and often vary with time. In such cases, an equalizer in the receiver with adaptive setting of parameters providing sufficient adjustable range might be employed to mitigate the signal degradation of the signal transmitted through the communications channel. The receive equalization might be through an analog equalizer (AEQ), a decision-feedback equalizer (DFE), or some combination of both. The shape of the data eye also changes due to equalization applied to input signal of the receiver. In some systems, equalization applied by a transmitter's equalizer further alters the shape of the eye from the ideal. The transfer characteristics of the AEQ and DFE are adaptive to enhance the performance of the SERDES receiver.
The DFE equalization optimizes for intersymbol interference (ISI) and opens up the vertical and horizontal data eye opening. In SERDES communication channels, DFE filtering is employed to cancel post-cursor ISI in the equalized channel's pulse response by subtracting the output of the DFE from an input signal. An adaptation processor implements a process or loop that attempts to minimize an error signal and thereby open further the vertical and horizontal data eye openings by continuously adjusting DFE parameters, here the DFE tap values.
To improve the performance of DFE-based receiver, the AEQ has a variable gain amplifier (VGA) used to control the input signal level, and a continuous-time analog equalizer (CTLE) used to compensate for linear, frequency-based distortions in the input signal to the receiver. The CTLE is generally implemented as an analog-based filter with at least one adjustable coefficient or peaking parameter that can at least partially compensate for linear distortions in the received signal. The above-mentioned adaptation processor also implements a process or loop that attempts to minimize an error signal and thereby open further the vertical and horizontal data eye openings by continuously adjusting various parameters of the AEQ, here the amount of gain provided by the VGA and the value of the CTLE coefficient.
The AEQ and DFE adaptation process might consume significant power even though the gain and coefficient parameters are changing little or not at all. However, to fully suspend the AEQ and DFE adaptation process runs the risk of degrading the performance of the SERDES receiver due to changes in the operating temperature and supply voltages the receiver is subject to that would normally require an update to the AEQ and DFE parameters. Thus, it is desirable to provide a technique for detecting receiver performance degradation due to temperature and operating voltage variations so that that the AEQ and DFE adaptation process may be suspended until needed, thereby reducing the power consumption of the SERDES receiver.